1. Field of the Invention
The present invention relates to a signal reproducing method for generating a reproduced signal using a magnetic oscillation device whose oscillation frequency is modulated by a magnetic field, a magnetic head using the magnetic oscillation device, and a magnetic recording and reproducing apparatus.
2. Description of the Related Art
A recording density of magnetic recording has improved at remarkable speed since appearance of a GMR head using a giant magneto resistance effect (GMR effect). A GMR element comprises a laminated layer having a sandwich structure of ferromagnetic layer, a nonmagnetic layer and a ferromagnetic layer. The GMR element is an element to detect a change of a relative angle between the magnetization directions of two ferromagnetic layers as a change of resistance value by applying an exchange bias to one ferromagnetic layer to fix magnetization and changing the magnetization direction of the other ferromagnetic layer by an external magnetic field, so-called an element using a magnetoresistance effect of a spin valve film.
A CIP-GMR element to detect a change of resistance when a current flows on the film surface of a spin valve film, and a CPP-GMR element to detect a change of resistance when a current flows in a direction vertical to the film surface of the spin valve film have been developed. It is conceived that the CIP-GMR element and the CPP-GMR element both have a magnetic resonance ratio (MR ratio) of around several %, and are able to deal to a recording density of about 200 Gbit/inch2. A TMR element using a tunnel magneto resistance effect (TMR effect) has been developed in order to deal with magnetic recording of a higher density.
The TMR element is composed of a laminated layer of ferromagnetic layer, insulator and ferromagnetic layer, and flows a tunneling current by applying a voltage between the ferromagnetic layers. The TMR element is an element which uses a phenomenon that magnitude of the tunneling current varies due to magnetization directions of the upper and lower ferromagnetic layers, and detects a change of a relative angle between magnetization directions as a change of a tunnel resistance value. An element that the MR ratio is around 100% at maximum is provided. Because the TMR element has a larger MR ratio than that of the GMR element, a signal voltage increases. However, there is a problem that a noise component due to a shot noise as well as a pure signal component increases, and thus a signal-to-noise ratio (SN ratio) is not improved.
The shot noise occurs due to fluctuation of a current generated by electrons passing a tunnel barrier in irregularity and increases in proportion to a square root of a tunnel resistance. Therefore, in order to acquire a necessary signal voltage with the shot noise being suppressed, it is necessary to thin the tunnel insulating layer to decrease the tunnel resistance. It is necessary to decrease the element size to a size similar to the recording bit according to increase of the recording density. Accordingly, it is necessary to decrease a junction resistance of the tunnel insulating layers according to increase of the recording density. The junction resistance not more than 1 Ω/cm2 is needed for the recording density of 300 Gbit/inch2, and a tunnel insulating layer having a thickness for two layers of atoms in terms of the film thickness of a Al—O (aluminum oxide film) tunnel insulating layer must be formed. A short circuit is apt to occur between the upper and lower electrodes with a decrease in the thickness of the tunnel insulating layer, resulting in decreasing a MR ratio. Therefore, manufacture of the element becomes difficult.
From the above reasons, it is estimated that a margin of the TMR element will be about 300 Gbit/inch2. Any of the above-mentioned elements utilizes a magnetoresistance effect (MR effect) in the general meaning. However, in late years, problems of magnetic white noise common to these magnetic resonance elements occur. Since this noise occurs due to heat fluctuation in minute magnetization unlike the electrical noise such as above-mentioned shot noise, it is thought that it becomes more dominant along with microminiaturization of the MR element and surpasses the electrical noise in the element corresponding to 200-300 Gbpsi.
It is necessary to use a free layer of small magnetic attenuation constant α for the magnetic white noise to be avoided. However, there is a problem that a reading speed decreases with a decrease in α. In late years, a magnetic sensor using a minute magnetic oscillation element is proposed as a measure for solving those problems (JP-A No. 2006-28609 (KOKAI)). If the minute magnetic oscillation element is used as a magnetic sensor to utilize a change of oscillation frequency due to a medium magnetic field, the high speed reading is possible even if the free layer of small magnetic attenuation constant α is used. However, there is a problem that S/N decreases since the minute magnetism oscillation element is poor in stability of oscillation frequency.
In the magnetic head and magnetic recording and reproducing apparatus using a magnetic oscillation element whose oscillation frequency varies by a magnetic field and generating a signal by detecting a frequency change of this magnetic oscillation element, it is difficult to reproduce a signal adequately, because the magnetic oscillation element is apt to fluctuate in phase.
The present invention is to provide a magnetic head and a magnetic recording and reproducing apparatus, which use a signal reproducing method and a magnetic oscillation element to reproduce a signal appropriately against a change of phase of the magnetic oscillation element and magnetism oscillation device and which allow a high speed response corresponding to high density recording and a high S/N ratio response.
According to the present invention, there is provided a signal reproducing method for reproducing a reproduced signal by detecting a phase difference between adjacent signals generated from a magnetic oscillation element whose oscillation frequency is modulated by a direction of a magnetic field.